Use the console to orient the camera plane exactly parallel to the x-y plane?
The question suggests that you are using the manipulator widgets or mouse to position and rotate your assets.  

Blender provides numerous input methods for performing transform operations upon assets.  

To specifically answer your question:
Typing (or pasting) the following into the console will center the camera 3 blender units above the world origin, and orient it so that it is looking directly at the x-y plane.

bpy.data.objects['Camera'].rotation_euler[0] = 0
bpy.data.objects['Camera'].rotation_euler[1] = 0
bpy.data.objects['Camera'].rotation_euler[2] = 0
bpy.data.objects['Camera'].location[0] = 0
bpy.data.objects['Camera'].location[1] = 0
bpy.data.objects['Camera'].location[2] = 3  


However, there are simpler methods for achieving the same results;
With the camera selected, you can input the numerical values directly into the input fields shown here
  

or even simpler; with the camera selected;
press [Alt + G] (to reset the camera position)
press [Alt + R] (to reset the camera rotation)
then drag on the blue manipulator arrow to raise the camera to any height you like.

Or you can set the camera to the view plane as suggested by NoviceInDisguise